Interface dilation. The overflowing cylinder technique.

A pure steady-state dilation of a liquid interface, either liquid-air or water-oil, can be accomplished far from equilibrium by means of the overflowing cylinder technique. The resulting dynamic surface tension data correlate well with characteristic parameters of processes like foaming, emulsification, and the spreading of droplets and thin liquid layers.Fundamental knowledge of the physical mechanism of operation of the overflowing cylinder technique is obtained by analyzing the relation between interface dilation and underlying bulk flow. Upon the addition of a surfactant the interface velocity increases considerably, since the propulsion mechanism changes from driven by the bulk flow to surface tension gradient driven.The surface rheological behaviour of the expanding interface is studied for various surfactant solutions. Generally practical systems give rise to a major increase in surface tension during interface dilation. The results are discussed in terms of the transport of surfactant components.The present findings explain why the overflowing cylinder technique is such a useful too[ for studying many practical processes which imply interface dilation far from equilibrium. Meanwhile, however, they urge a reconsideration of the meaning of the surface dilational viscosity